1. Field of the Invention
The present invention relates to battery separators. More particularly, the invention relates to battery separators made from 4-methyl-1-pentene (co)polymer and excellent in heat resistance properties, strength and elasticity, and to lithium ion batteries having the separators.
2. Related Art
Separators in lithium ion batteries must separate positive electrodes and negative electrodes to prevent short circuits, and must allow passage of electrolytes or ions. Porous films and nonwoven fabrics of resins are known separators. The lithium ion battery separators are also required to exhibit stable properties even at high temperatures such as in charging, and therefore high heat resistance is desired.
Nowadays, polyethylene sheets manufactured by a drawing process or phase separation method have been in practical use as the lithium ion battery separators composed of porous films, and porous films of ultra high molecular weight polyethylene are known. Porous films of high-melting point polyolefins, polyethyleneterephthalate and nylons have been proposed for enabling uses at high temperatures, but manufacturing porous films from high-melting point resins is difficult.
On the other hand, nonwoven fabric separators are suited for mass production and possess advantageous high porosity and lightweight, so that various nonwoven fabrics and separators using thereof having high heat resistance and small pore diameters have been studied.
For example, JP-A-2002-124238 proposes a high heat resistance separator made of polymethylpentene meltblown nonwoven fabric. Further, JP-A-2003-142064 discloses a separator that is thin but sufficient in mechanical strength and possesses superior shape retention at elevated temperatures, being composed of a porous fiber sheet that has a specific thickness and porosity and comprises a branched α-olefin polymer such as 4-methyl-1-pentene polymer.
These nonwoven fabric separators, however, are still insufficient in strength when subjected to tension during battery manufacture such as wound-type batteries, resulting in width reduction that is a shrinkage of sheet in a direction perpendicular to the pulling direction. The reduced width of the separator leads to insufficient insulation between both battery electrodes in the area, and may cause short circuits. Further, the separator stretched in the pulling direction is enlarged in pore size and the possibility of micro-short circuits is increased.
The present inventors have made intensive studies in view of the above circumstances, and have found that meltblown nonwoven fabrics have microscopic variation of basis weight in different parts. This variation changes little even after the meltblown nonwoven fabrics are pressed or embossed with metal rolls or press plates. Accordingly, the meltblown nonwoven fabrics or pressed products thereof used as separators cause microscopic nonuniform passage of electrolytes or ions in different parts. Therefore, it is expected that more uniform pore diameters and diameter distribution will reduce the internal resistance.
There has therefore been a need for a separator that can exhibit heat resistance and mechanical strength enough to retain its shape during manufacture and use of lithium ion batteries, and has excellent insulating properties, fine pores and high productivity.